The present invention relates generally to analytical instruments, and specifically to instruments and methods for thin film analysis using X-rays.
X-ray reflectometry (XRR) is a well-known technique for measuring the thickness, density and surface quality of thin film layers deposited on a substrate. Conventional X-ray reflectometers are sold by a number of companies, among them Technos (Osaka, Japan), Siemens (Munich, Germany) and Bede Scientific Instrument (Durham, UK). Such reflectometers typically operate by irradiating a sample with a beam of X-rays at grazing incidence, i.e., at a small angle relative to the surface of the sample, near the total external reflection angle of the sample material. Measurement of X-ray intensity reflected from the sample as a function of angle gives a pattern of interference fringes, which is analyzed to determine the properties of the film layers responsible for creating the fringe pattern. The X-ray intensity measurements are commonly made using a position-sensitive detector, such as a proportional counter or an array detector, typically a photodiode array or charge-coupled device (CCD).
A method for analyzing the X-ray data to determine film thickness is described, for example, in U.S. Pat. No. 5,740,226, to Komiya et al., whose disclosure is incorporated herein by reference. After measuring X-ray reflectance as a function of angle, an average reflectance curve is fitted to the fringe spectrum. The average curve is based on a formula that expresses attenuation, background and surface roughness of the film. The fitted average reflectance curve is then used in extracting the oscillatory component of the fringe spectrum. This component is Fourier transformed to find the film thickness.
U.S. Pat. No. 5,619,548, to Koppel, whose disclosure is incorporated herein by reference, describes an X-ray thickness gauge based on reflectometric measurement. A curved, reflective X-ray monochromator is used to focus X-rays onto the surface of a sample. A position-sensitive detector, such as a photodiode detector array, senses the X-rays reflected from the surface and produces an intensity signal as a function of reflection angle. The angle-dependent signal is analyzed to determine properties of the structure of a thin film layer on the sample, including thickness, density and surface roughness.
U.S. Pat. No. 5,923,720, to Barton et al., whose disclosure is incorporated herein by reference, also describes an X-ray spectrometer based on a curved crystal monochromator. The monochromator has the shape of a tapered logarithmic spiral, which is described as achieving a finer focal spot on a sample surface than prior art monochromators. X-rays reflected or diffracted from the sample surface are received by a position-sensitive detector.
Various types of position-sensitive X-ray detectors are known in the art of reflectometry. Solid-state arrays typically comprise multiple detector elements, which are read out by a CCD or other scanning mechanism. Typically, each element accumulates photoelectric charge over a period of time before being read out and therefore cannot resolve the energy or number of incident X-ray photons. XRR systems known in the art that are based on such arrays simply record the total integrated radiation flux that is incident on each element. The signals at low angles, below the total external reflection angle, are usually much stronger than the signals above this angle. A ratio of 105 to 107 in photon flux between 0xc2x0 and 3xc2x0 reflections is typical. The dynamic range of array detection systems known in the art is substantially smaller than this ratio. Consequently, high-order fringes at higher incidence angles cannot generally be detected. Photon counting sensitivity is needed in order to measure the weak signals at these angles.
A further drawback of X-ray thin film measurement systems known in the art is their lack of spatial resolution. X-ray optics, such as the above-mentioned curved monochromators, are capable of focusing an X-ray beam to a spot diameter below 100 xcexcm. When the beam is incident on a surface at a low angle, below 1xc2x0, for example, the spot on the surface is elongated by more than 50 times this diameter. A measurement that is made under these circumstances provides only an average of surface properties over the entire elongated area. For many applications, such as evaluating thin film microstructures on integrated circuit wafers, better spatial resolution is required.
Although the present patent application is concerned mainly with systems in which a sample is irradiated by a monochromatic beam, other methods for X-ray reflectometry are also known in the art. One such method is described, for example, in an article by Chihab et al., entitled xe2x80x9cNew Apparatus for Grazing X-ray Reflectometry in the Angle-Resolved Dispersive Mode,xe2x80x9d in Journal of Applied Crystallography 22 (1989), p. 460, which is incorporated herein by reference. A narrow beam of X-rays is directed toward the surface of a sample at grazing incidence, and a detector opposite the X-ray beam source collects reflected X-rays. A knife edge is placed close to the sample surface in order to cut off the primary X-ray beam, so that only reflected X-rays reach the detector. A monochromator between the sample and the detector (rather than between the source and sample, as in U.S. Pat. No. 5,619,548) selects the wavelength of the reflected X-ray beam that is to reach the detector.
It is an object of the present invention to provide improved methods and systems for X-ray analytical measurements, and particularly for measurements of thin film properties.
It is a further object of some aspects of the present invention to provide systems for X-ray reflectometry with enhanced dynamic range.
It is still a further object of some aspects of the present invention to provide systems for X-ray microanalysis with enhanced spatial resolution.
It is yet a further object of some aspects of the present invention to provide systems for measurement of X-ray reflectance with enhanced signal/noise ratio.
In preferred embodiments of the present invention, a system for X-ray reflectometry is used to determine properties of thin films on the surface of a sample, typically a semiconductor wafer. The sample is irradiated by a monochromatic beam of X-rays, which is focused to a small spot size on the surface of the sample. X-rays reflected from the surface are incident on a detector array, preferably a CCD array, each detector element in the array corresponding to a different angle of reflection from the surface. Charge stored by the detector elements is clocked out of the array to a processor, which analyzes the charges to derive a fringe pattern, corresponding to the intensity of X-ray reflection from the surface as a function of angle. The X-ray source, optics and processing circuitry in the system are arranged to achieve a high signal/noise ratio and high dynamic range, whereby high-order fringes are plainly apparent in the reflected signal. The processor analyzes the fringe pattern based on a physical model of thin film properties, including density, thickness and surface roughness. The high dynamic range enables the system to determine these properties accurately not only for the upper thin film layer, but also for one or more underlying layers on the surface of the sample.
In some preferred embodiments of the present invention, the sample is irradiated using a pulsed X-ray source, as is known in the art. The detector array is gated in synchronization with the pulsed source, preferably by clearing the charge stored by the array elements just before the source is fired, and then reading out the elements immediately after the excitation pulse. In this manner, the integrated contribution of steady-state background effects, such as thermal noise, to the output of the detector array is reduced in proportion to the gating duty cycle of the array. On the other hand, as long as the average power of the X-ray source is the same in pulsed mode as in conventional constant-wave (CW) operation, the same total signal output from the detector array is maintained. In this manner, a significant improvement is achieved in signal/noise ratio of the system.
Although preferred embodiments of the present invention are directly mainly toward enhancing X-ray reflectometric measurements on thin films, and particularly on semiconductor wafers, the principles of the present invention can similarly be used in other applications of X-ray reflectometry, as well as in other types of radiation-based analysis.
There is therefore provided, in accordance with a preferred embodiment of the present invention, reflectometry apparatus, including:
a pulsed X-ray source, adapted to irradiate a sample with a sequence of pulses of radiation over a range of angles relative to a surface of the sample;
an array of detector elements, positioned to receive the radiation reflected from the sample over the range of angles and to generate a signal indicative of respective charges accumulated by the detector elements due to photons of the radiation that are incident on the elements; and
timing circuitry, coupled to the array so as to cause the charges to be cleared from the detector elements immediately before each of the pulses in the sequence, and to cause the signal from the elements to be sampled shortly after each of the pulses.
Typically, the sample includes one or more thin film layers, and the reflected radiation is characterized by an oscillatory variation of intensity as a function of the angles due to the thin film layers, and the array of detector elements is adapted to detect the oscillatory variation. Preferably, the apparatus includes a processor, which is coupled to receive the signal from the array of detector elements and to analyze the oscillatory variation to determine one or more properties of the one or more thin film layers. Most preferably, the processor is adapted to estimate, responsive to the respective charges, a number of the photons that was incident on each of the elements.
Further typically, the detector elements are characterized by a background current, which causes a noise charge to be accumulated by the detector elements irrespective of the radiation, and by causing the charges to be cleared from the detector elements, the timing circuitry is operative to reduce the noise charge in the signal sampled from the elements shortly after each of the pulses. In a preferred embodiment, the array of detector elements includes a charge coupled device (CCD).
Preferably, the timing circuitry is coupled to synchronize clearing of the charges from the detector elements and sampling of the signal therefrom with the pulses from the X-ray source.
There is also provided, in accordance with a preferred embodiment of the present invention, a method for reflectometry, including:
positioning an array of detector elements to receive radiation reflected from a sample and to generate, responsive to the radiation, a signal indicative of respective charges accumulated by the detector elements due to photons of the radiation that are incident on the elements;
irradiating a sample with a sequence of pulses of the radiation over a range of angles relative to a surface of the sample;
clearing the charges from the detector elements immediately before each of the pulses in the sequence; and
sampling the signal from the elements shortly after each of the pulses so as to determine a pattern of the reflected radiation over the range of angles.
The present invention will be more fully understood from the following detailed description of the preferred embodiments thereof, taken together with the drawings in which: